In a dry etching semiconductor device fabricating process, a high selectivity, that is, faster etching of a thin film than of the underlying substance or mask material such as photoresist, is required. Furthermore, a high anisotropy, that is, faster etching perpendicular than parallel to the surface of a thin film is required. Furthermore, a low likelihood of damage of the surface of the thin film etched is required. The requirements for etching, such as high selectivity, high anisotropy, and low likelihood of damage, increase with an increasing degree of device integration.
Prior art dry etching apparatus utilizing a high frequency discharge plasma an RF discharge or microwave discharge are widely adopted in semiconductor device fabrication processes for fine pattern processing ability and mass producibility.
FIG. 6 is a schematic diagram showing a prior art dry etching apparatus using an RF discharge plasma disclosed in "Opto Plasma Processing", Kazuo Akashi, Shuzou Hattori, Osamu Matsumoto (Nikkan Kogyo Shinbun-sha, Tokyo, 1986), Chapter 10, "Basis of Plasma Etching" (by Yukio Yasuda).
In FIG. 6, reference numeral 1 designates a substrate to be etched, reference numeral 2 designates a first parallel plate electrode on which the substrate 1 is placed, reference numeral 3 designates a second parallel plate electrode arranged confronting to the first parallel plate electrode 2. Reference numeral 4 designates a plasma reaction vacuum container in which the first parallel plate electrode 2 and the second parallel plate electrode 3 are provided. Reference numeral 5 designates a vacuum exhausting means for exhausting internal gas of the container 4. Reference numeral 6 designates a RF power applying means for applying RF power between the first parallel plate electrode 2 and the second parallel plate electrode 3. Reference numeral 7 designates a gas bomb in which etching gas A which is to be supplied to the plasma reaction container 4 is contained. Reference numeral 8 designates a gas flow rate control means for adjusting the flow rate of etching gas from the gas bomb 7 to the container 4. Reference numeral 9 designates a pressure meter for observing the pressure of the etching gas in the container 4. Reference numeral 10 designates an RF discharge plasma generated between the first parallel plate electrode 2 and the second parallel plate electrode 3.
As material to be etched, underlying substance, and photoresist on the substrate 1 to be etched, polycrystalline silicon, silicon dioxide, and polymethyl methacrylate are respectively used and as etching gas A in the gas bomb 7, a mixture of chlorine (Cl.sub.2) and argon (Ar) is used. The frequency of the RF power applying means 6 is, for example, 13.56 MHz.
A description is given of the operation hereinafter.
First of all, the substrate 1 to be etched is placed on the first parallel plate electrode 2 in the plasma reaction vacuum container 4, and thereafter vacuum exhaustion in the container 4 is carried out by the vacuum exhaustion means 5. Subsequently, the flow rate of the etching gas A supplied to the container 4 from the gas bomb 7 is adjusted and established by the gas flow rate control means 8, and further the gas pressure in the container 4 is observed by the pressure meter 9 and the gas exhaustion speed of the vacuum exhaustion means 5 is adjusted and the etching gas pressure in the container 4 is established.
Subsequently, when RF power is applied to the first and second parallel plate electrodes 2 and 3 by the RF power applying means 6, the etching gas in the plasma reaction container 4 is ionized to be in a plasma state by the high frequency glow discharge generated between the electrodes 2 and 3 and reactive weakly ionized plasma 10 is generated. By the neutral atomic radicals or atomic ions generated or neutral molecular radicals or molecular ions constituted by those combinations, the substance on the substrate 1 is etched.
Here, when a mixed gas of Cl.sub.2 and Ar is used as the etching gas A, Cl is generated as neutral atomic radical, Cl.sup.+ and Ar.sup.+ are generated as atomic ions, and Cl.sub.2.sup.+ is generated as molecular ions. In these examples, there are no molecular radials. In addition, the etching gas pressure in the plasma reaction container 4 is 0.01 to 1 Torr, 3.times.10.sup.14 to 3.times.10.sup.16 cm.sup.-3 gas density, and the plasma density is approximately 10.sup.9 to 10.sup.11 cm.sup.-3.
FIG. 7 is a schematic diagram showing a mechanism in which the substance on the substrate placed on the electrode in the plasma is etched in the prior art dry etching apparatus using an RF discharge plasma. In this discharge plasma, there exists a plurality of kinds of neutral atoms and molecules having various energies and their ions and electrons and photons (radiation) having various energies, but the particles directly contributing to the etching of the substance on the substrate are atomic and molecular ions and neutral atomic and molecular radicals in the space charge region (sheath) produced in the neighborhood of the substrate.
In this sheath region, positive ions having positive charges are likely to be accelerated in the substrate direction by the sheath voltage of about 100 to 1000 V (plasma potential+self bias voltage of the electrode) and be vertically incident on the surface of substrate. However, the neutral radicals do not respond to the sheath voltage and they are isotropically incident on the surface of substrate by the thermal disordered movement at approximately 500.degree. to 1000.degree. K.
In such dry etching apparatus, the selectivity of etching, that is, the ratio of the etching speed of the thin film to be etched and the etching speed of the underlying substance or the mask material such as photoresist is mainly obtained by the difference in the chemical reactivity against the respective materials of neutral radicals. The chemical etching due to only neutral radicals isotropically incident on the surface of substrate is isotropical.
On the other hand, the anisotropy of etching, that is, the ratio between the etching speed perpendicular to the surface of thin film to be etched and the etching speed parallel to the surface is obtained mainly by the directionality of movement against the surface of ions which are accelerated by the sheath voltage and incident vertically to the surface of substrate, and the physical sputtering due to ions is non-selective to the respective material and the surface layer of the thin film to be etched is damaged. Practically, etching of substance on the substrate proceeds by the competition process of these neutral radicals and ions which is called as an ion assist process.
The prior art dry etching apparatus using discharge plasma is constituted such that anisotropic of etching is obtained only by the directionality of the movement of ions which are accelerated by the sheath voltage and vertically incident on the substrate surface in the space charge region (sheath) produced in the neighborhood of the substrate. In order to enhance the anisotropy of etching, the sheath voltage or further incident energy of ions into the substrate has to be enhanced, and as a result, since high anisotropy is desired, the selectivity is much lowered, thereby resulting in an increase in damage.